Electrical devices and methods are known for providing electric stimulation to tissue and cells for desired therapeutic effects such as the type described in Hansjurgens, U.S. Pat. No. 5,573,552 the disclosure of which is hereby incorporated by reference. Generally these prior devices and methods are adapted to stimulate tissue and bone cells to promote healing and the like, to stimulate muscle contractions to provide a therapeutic effect thereto and to stimulate or block the transmission of signals by nerves to, for example, decrease or block pain. Also if heating is desired, the current applied to the tissue can be increased to induce heating of the cells also to produce a therapeutic effect or to destroy cells such as tumors or the like.
A drawback of the devices and methods suggested by Hansjurgens Pat. No. 5,573,552 (the '552 patent) is that their operation is premised upon using a constant amplitude while varying the frequency of the potential applied to the cell or nerve in a predetermined, ramping fashion. As described in the '552 patent, corner frequencies are selected, a lower comer frequency and a higher, upper corner frequency. These comer frequencies are selected to span the frequency (at the constant amplitude) which triggers the desired stimulus response (e.g. muscle contraction) in the tissue or nerve. Thus in most instances the upper corner frequency exceeds the frequency necessary to trigger the desired response. With a constant amplitude, the frequencies are modulated in a linear, ramping, sweeping, fashion between the lower and upper corner frequencies crossing, during their traversal up and down between these frequencies, exceeding the action potential frequency eliciting the stimulus response such as a muscle contraction which may be at, for example, 1500 Hz. It has been found that once a stimulus response has been elicited, that amplitude must be increased or frequency reduced to elicit another response. This phenomena is believed to be based upon the target such as a muscle, becomes conditioned not to respond to the same magnitude of stimulus. Thus the ramping approach described above has worked with a constant amplitude since frequency is varied during the ramping sweep of frequencies to trigger the stimulus response during the sweep and the upper corner frequencies have been selected to far exceed the value necessary to trigger the response. Thus even though the tissue becomes conditioned, the increasing values will ultimately elicit the response.
With reference to FIG. 1, there is shown the relationship between tissue impedance A, current delivered B, power C versus frequency. As can been seen the tissue impedance Z has an inverse relationship to frequency, i.e. decreases as frequency increases. Power P and current I have a direct, non-linear, relationship to frequency. For current I, if the voltage remains constant, the reduction in impedance Z with the increase in frequency, increases the current I delivered. Similarly, since power P is the product of voltage and current I, if voltage is held constant the power P will increase proportionately with the current I. Another relationship that is important to understand is that energy is the product of power and the length of time the power is delivered. FIG. 2 shows the relationship between energy and time for various frequencies. Line D represents, for example, a frequency of 20,000 Hz, line E for a frequency of 10,000 Hz, line F for a frequency of 5000 Hz and line G for a frequency of 2500 Hz.
Because the amplitude of the prior art system as described in the above patent is held constant (frequency is varied), there is no means to control the amount of energy (heat) which would be delivered without altering the lower and upper corner frequencies, i.e. starting points of the ramping sweep, or frequency step (the increase in frequency), frequency sequence or dwell time, i.e. how long the power is delivered at that frequency. Thus, one cannot alter one desired parameter such as energy without adjusting one or more of the other parameters. For example, one could not reduce the amount of energy delivered to consume less power, without, for example, lowering the corner frequencies and thus not, perhaps, obtaining the stimulus threshold required for stimulation of the tissue or nerve.
Furthermore these prior devices do not permit the operator to select a delivery protocol such as one to deliver multiple signals to elicit a stimulus response during a sweep cycle while minimizing the power required. Thus one could not heretofore select during a sweep period that a first stimulus response by induced during a first portion of the sweep, again near the middle of the sweep and again near the end of the sweep while the intervening intervals during the sweep are selected to minimize the power required.
There is a need for a device and method which overcomes the aforementioned drawbacks, which enables the selection of one or more events, such as a stimulation response, to occur during a sweep period and which otherwise selects parameters for the sweep corresponding to a desired, overall, protocol be it conservation of power, heating or the like.